John Dunbar
correct about everything
Maybe we'll find out why she did it? Lyanna wasn't stupid. The fact that she knew Robert would kill Jon/Aegon tells me she's aware of her actions.
Robert would have come after Rhaegar regardless because he would have seen Rhaegar's action of giving the blue roses to Lyanna as the disrespecting his honour.
Bethroted is basically a formal engagement to be married. She was Roberts. Period.
The war would have happened, because the Targaryan prince started it and brought dishonour to House Stark and Baratheon.
Aerys just made it much worse, much faster.
there's no guarantee the starks or jon arryn would have gone to war for just a kidnapping, and especially not an elopement. robert might have (though he did not until actual deaths happened and his life was at risk), but he could not have done it alone.